High voltage electrodes



VOLUME RESISTIVITY (Ohm Cm) April 1963 J. .1. MURRAY 3,087,056

HIGH VOLTAGE ELECTRODES Filed July 14, 1961 2 Shets-Sheet 1 HIGH VOLTAGE SUPPLY I00 300 500 INVENTQR. TEMPERATURE JOSEPH J. MURRAY ATTORNEY.

April 23, 1963 J. J. MURRAY HIGH VOLTAGE ELECTRODES 2 Sheets-Sheet 2 Filed July 14, 1961 momDOm FZmmmDo INVENTOR.

JOSEPH J. MURRAY mokowkwo ATTORNEY.

Unitd States Patent Ofifice 3,087,0- Patented Apr. 23, 1963 3,087,0S6 HIGH VGLTAGE ELECTRGDES Joseph J. Murray, Oakiand, Califl, assignor to the United States of America as represented by the United States Atomic Energy Commission Filed .luiy 14, 1961, Ser. No. 124,242 7 Claims. (Cl. 25041.9)

The present invention relates to high voltage electrode structure and more particularly to means for suppressing sparking between spaced apart electrodes whereby high intensity electric fields may be maintained therebetween.

The present invention was originally developed to produce a high intensity electric field in a velocity spectrometer. This form of spectrometer is used in conjunction with momentum analyzing systems to produce angular and spatial displacement of particles of different mass which occur in secondary charged particle beams produced by accelerators such as proton synchrotrons of the class providing energies in the order of several billion electron-volts. In the velocity spectrometer, a magnetic field is crossed with the electric field to effect the separating function, the magnetic field impelling particles in one direction and the electric field impelling the particles in the opposite. For particles with a certain velocity the two forces just balance and only such particles appear at the terminal end of the spectrometer. The intensity of the electric field which can be maintained in the spectrometer is a factor which limits the maximum particle energy at which the separating function will be effective. The present invention allows the electric field intensity to be increased by two to three times with a corresponding (though not linearly related) increase in beam particle energy or, in appropriate cases, a reduction in the size of the apparatus for a given particle energy.

While the invention has been found highly useful in the above described spectrometer application it is by no means limited to such usage and is equally applicable in diverse other forms of equipment in which high electrical gradients are required. In general the invention allows much stronger electric fields to be maintained between electrodes of given spacing than has heretofore been feasible.

The electric fields to be discussed herein are of the type produced between two spaced apart electrodes separated by a vacuum or near vacuum with pressures usually less than 10- millimeters of mercury. A direct current voltage is connected between the electrodes to provide an electric field within the vacuum therebetween. With this arrangement, it has generally been possible to hold more intense fields between closely spaced electrodes than across more widely spaced electrodes owing to the greater likelihood of a sustained are occurring with greater spacmg.

Considering now the structural characteristics of the present invention, the limiting class of vacuum sparks in the above described electrode arrangement (but not necessarily all sparks) appears to involve in the incipient stage the emission of electrons from very small areas of the surface of the cathode electrode at the eventual spark sites. On this basis it has now been found that if the volume (specific) resistivity of the cathode is high enough, and certain other conditions are met, the degenerative effect of the local voltage drop at the electron emitting sites will prevent instabilities which would otherwise cause sparks. Concurrently, the potential at other points on the surface of the cathode is essentially unchanged by the process so that the major portion of the electric field is undisturbed by the incipient spark and extinguishing thereof. Even though the ultimate nature of sparks may differ widely depending on a diversity of circumstances, if

an incipient property common to all, such as electron emission from the cathode, is controlled, sparking is effectively suppressed.

To meet the above requirement the cathode electrode material may have a specific resistivity in the range from 1 to 10 ohm-centimeters.

To utilize the resistive effect described above, certain additional physical conditions must be fulfilled which limit the selection of suitable cathode materials. One consideration is the time constant for approach to equilibrium charge distribution on the cathode surface. For an isolated smooth surface this time constant is essentially the product of volume resistivity times capacitivity. To obtain sufiiciently conductor-like behavior for the cathode material for static conditions, time-constants of the order of one millisecond (10" sec.) are acceptable.

A second consideration in selecting the cathode material is the effective quenching of cold electron emission from the surface of the material by inducing a backward electric field through the material which must be comparable to the field strength causing the cold emission. Such backward electric field may be of the order offer greater than 10 million volts per centimeter.

A third consideration is the stability of the cathode material including thermal stability wherein the cathode does not vaporize, dielectric stability wherein there is no breakdown with the internal electric fields encountered, and mechanical stability wherein the material Will not be ruptured by the physical stresses imposed by the high potentials present. A still further desirable consideration is that the above properties should be realized in a dynamically stable steady state. As Will hereniafter be discussed in greater detail, the most commonly available materials which meet the foregoing requirements are various types.

of glass, for example soda-lime glass. Most glasses, however, must be heated to a temperature as high as a few hundred degrees centigrade in order to lower the resistivity into the desired range.

Accordingly the invention, in its preferred form comprises a pair of spaced apart electrodes having facing parallel flat inner surfaces and a high voltage source connected to establish a high potential difference between the electrodes. Disposed against the inner surface of the more negative electrode is a layer of glass, or other material having the characteritics herein discussed, which layer effectively constitutes the negative electrode.

It is therefore an object of the present invention to provide a means for creating more intense steady-state electrical fields than have heretofore been generally obtainable.

It is another object of the present invention to provide an electrode structure for establishing a very high gradient electric field, said structure acting to suppress incipient arcing across the field.

It is yet another object of the present invention to provide an electrode structure which inhibits the emission of electrons from the electrode in vacuum thereby allowing an increase in the intensity of the electric field associated with the electrode.

It is still another object of the invention to provide more intense electrical fields to facilitate the control and analysis of charged particle beams of energies higher than that heretofore conviently handled.

It is yet another object of the invention to provide a high voltage electrode material having a combination of electrical characteristics which suppress incipient sparks whereby an extremely high voltage may be held on said electrode.

The invention will be better understood by reference to the accompanying drawings of which:

FIGURE 1 is a perspective view of a pair of high positive electrode 11.

voltage electrodes with energizing means for providing an extremely intense electrical field, a portion of the electrodes havingbeen broken away to show a cross section thereof and to permit an indication of the resistive current paths associated with are suppression,

FIGURE 2 is a graph of the volume resistivity as a function of temperature for materials usable for the negative-electrode in the. structure of FIGURE 1,

FIGURE 3 is an equivalent circuit diagram of the apparatus of FIGURE 1, and

FIGURE 4 is a simplified perspective view showing components ofv a velocity spectrometer illustrating a typical usage of the invention.

Referring now to FIGURE 1, there is shown a fiat generally rectangular positive electrode 11 formed of a highly conductive material such as stainless steel. A flat negative electrode 12 comprised of glass, or a comparable material,..having a fairly high specific resistivity which may be of the order. of from lto 10G ohm-crn., is disposed in, a spaced apart parallel relationship with the A backing plate 13 of stainless steel or other highly conductive material, contacts the backsurface of the negative glass electrode12on the side opposite thepositive electrode 11, the backing plate 13 beingparallel to electrode 12 and disposed thereagainst. The edges of the electrode members 11, 12 and 13 are preferably roundedto suppress sparking and all exposed surfaces are polished to reduce irregularities from which eleptrpn emission would tend to occur. To provide for maximum voltage holding ability, the space between electrodes ll and lz is evacuated and accordingly a suitable vacuum envelope, indicated schematically at 14, surrounds the electrodes 11 and 12. A high voltage power supply 1 6. h as a positive terminal connected to the positive electrode 11 and has a negative terminal connected to the backing plate 13 of. the negative electrode 12.

I When a high voltage from source 16 is applied, there isavery intenseelectric field between the negative electrode 12'and the positive electrode 11 as indicated by electric field lines 15 in FIGURE 1. Since the backing plate 13 contacts nearly all of one surface of the negative electrode 12, there is provided a short uniform path for the positive charge carriers from the surface of the negative electrode 12 facing the positive electrode 11, although such uniformity is not essential to the functioning of the invention in all instances. If an incipient spark site should for any reasonexist at a point 17 on the negative' electrode 12, the initial effect will be electron emission as indicated schematically at 18. The necessary supply of electrons 18 for initiatingand sustaining the spark must ilow through the fairly high resistance of the negative electrode 12 along paths indicated schematically in FIGURE 1 by resistances 19. The voltage drop caused by this electron current through the high resistance of the negative electrode 12 to point 17 causes the voltage at point 17 to approach that of positive electrode 11, thus reducing the voltage at the spark site 17 and quenching the electron emission.

Since the threshold for significant sparking between electrodes 11 and 12 is raised by the foregoing effect, a much higher potential difierence may be maintained between the electrodes relative to conventional metal electrodes. Increases of the potential ditference by factors of from two to three have been realized and electrical fields having intensities up to 2.5 million volts per centimeter have been maintained.

With respect to the selection of a suitable material for the cathode electrode 12, one consideration is the time constant for approach to equilibrium charge distribution on the cathode surface. For an isolated smooth surface this time constant, 7- is given by the product of volume resistivity, p, and capacitivity, e, when inductive effects and more complex relaxation phenomena are ignored. In order to realize conductor-like behavior in a static i sense an upper limit for 'r of the order of a millisecond is preferred. Thus a first condition is If there is to be any effective quenching of cold emission, the backward :field, E, induced on the vacuum side of the cathode surface at the emitting site must be comparable to the field strength required to cause appreciable cold emission, or in order to magnitude 10" volts/cm. A second preferred condition is, then,

where i is the emission current density and s is the capacitivity of vacuum 8.85 10- sec. ohm-cm.

Finally, it is required as a third condition (3) that stability of the following types be provided: (a) thermalno appreciable cathode vaporization, (b) dielectric-no breakdown with internal electric fields of the order (c) mechanicalno rupture for stresses of the order (d) dynamic stability, where dynamic stability implies stability in temporal sense, as determined by the dynamical relationship among the various parameters of the system in the absence of more or less violent and discontinuous instabilities of the former types. The system involved here is so complex thatonecan do little more analytically than recognize the need for dynamic stability, and condition (3d) therefore cannot .very well influence the choice of material. Furthermore, although its fulfillment is important, condition (3c).woul d not be expected to impose severe restrictions on,.the choice of material, since the stress associated with .a field of 10 volts/cm. is only about 6-00 psi. Condition (3b), however, suggests the choice of a materialwith high dielectric strength and high capacitivity. In addition, condition (3a) together with condition (2) can be used to approximate ,a lower limit for p asfollows:

Given a critical temperature, T (certainly less than the melting temperature of the cathode material), and assuming that ohmiclosses are dissipated solely by thermal conductivity, it can be demonstrated with the aid of simplifying-models or dimensional analysis that the relationship between E and the other parameters of the system is dominated by the term:

where AT T, I T,, ,is the temperature rise at the emitting site on the cathode surface, r is the radius of the site, and k is thermal conductivity. C is a numerical factor of order unitywhich depends on the geometrical details of the site. Setting C equal to unity and using condition (2), E v./ cm., one gets an order-ofmagnitude estimate of a lower limit p,

in ohm-cm for r in cm. and kAT in watt cmr There is experimental evidence indicating that the effective 11 for tungsten is of the order 10" to 10- cm., and forpresent purposes this may be assumed to be generally true for other materials under consideration. For metals semiconductors with I z melting, kAT varies between 10 and 10 watts/cm, so that using 10 as a representa-tive value for 6/6 gives the condition for metals and semiconductors: pill)- to 10 ohm-cm. This is an unattainable range of resistivity for metals and is possible but marginal for semiconductors; that is, these materials 5 would probably melt before an appreciable back field, E, developed.

For glasses, on the other hand, with T T the value of kAT is about 2 watts/cm., so that with 6/6 again set equal to 10; Zl to 10 ohm-cm. This is a lower limit of resistivity always exceeded by a wide margin in glasses but at the same time much less than the upper limit, ZlO /e IQ ohm-cm. set by condition (1), thus leaving a wide acceptable range of values of p which can be attained in heated glass. In FIGURE 2 nominal values of volume resistivity as a function of temperature for soda-lime glass, Pyrex, and fused quartz are indicated. Condition (3b) is also satisfied by these glasses even though their capacitivities are not especially high, ranging from 3.5 for fused quartz to about 10 for soda-lime glass, since the intrinsic dielectric strength for each is so very high about 5-10 v./ cm. These considerations make heated glass a preferred material for the negative electrode 12.

Tests have shown that the electric field starts at the surface of the glass electrode 12 and not at the backing plate 13, indicating that the glass is functioning in a manner resembling that of a plate of a capacitor and does not act as a dielectric between the backing plate and the positive electrode 11. In fact, the backing plate 13 may be replaced by a point contact and in the absence or" appreciable current flow the electric field is essentially unaffected.

Referring now to FIGURE 3, the operation of the invention will be further clarified by reference to an equivalent circuit analog of the electrode structure which is shown for the condition where no sparking is occurring. A capacitor 51 is equivalent to the vacuum gap between the positive and negative electrodes 11 and 12 of FIG- URE 1 while the negative electrode 12 may be shown by a capacitor 52 and resistor 53 in parallel, the parallel combination being connected in series with the capacitor 51. The capacitor 52 and resistor 53 combination will typically have a time constant of the order of 1-l0 seconds. A potential from the source 54- applied to the R-C combination appears entirely across the vacuum gap capacitor 51. However, when a spark occurs across the vacuum gap, the potential is almost entirely across the parallel resistor 53 and capacitor 52. A time in tie order of milliseconds is required for the redistribution of various charge carriers in the glass electrode, therefore the invention responds to DC voltage variations up to the kilocycle range.

In the operation of the invention at voltages in excess of 300 kv. a new phenomenon occurred, one not observed at lower voltages, namely an ion-exchange process. With a sudden onset, identically shaped luminous patches appeared on the electrodes Ill and 12, stable at threshold but rapidly becoming intense and developing into unstable discharges as the potential was raised only a few percent above threshold. The threshold voltage for onset of the ion exchange increased at first when the system was forced to discharge repeatedly, but was observed to stabilize eventually at about 400 kv. almost independently of the spacing of the electrodes for spacing between 1.5 and 7 cm.

Identification of the discharge as an ion-exchange process is based on the following observations. The luminous patches, sometimes covering an area of 1 cm. or more, were most intense and, in spite of a certain amount of movement, tended to occur mainly at depressed areas of the anode electrode 11 which was not a particularly flat surface, but with relative deviations from flatness of as much as 2 mm. This is a natural consequence in an ion exchange if the secondary ions are emitted with essentially zero velocity. A striking demonstration of the same effect occurs when the electrodes 11 and 12 are not parallel. In this case a single luminous spot may appear on each electrode at the onset of a discharge, but as the voltage is r-aised a number of regularly spaced patches de- 6 velop, spreading out along a line directed toward the widest part of the gap.

The luminous patches are unafiected by magnetic fields of several hundred gauss transverse to the electric field, which would be expected to cause noticeable distortion or motion of the patches if electrons where involved as significant agents in effecting the exchange (electrons are evidently involved at least in a passive role, as evidenced by high X-ray yields).

After prolonged operation (several hours) with a stable ion exchange in progress, inspection has revealed appreciable erosion of the cathode elect-rode 12 in the vicinity of the luminous patches, sometimes without any other damage at all. Occasionally currents in excess of 1 ma. can flow before an ion-exchange discharge becomes unstable. At currents greater than about 200 ,ua., however, cathode damage in addition to erosion usually occurs, apparently as a result of local heating which causes the surface of the glass to chip.

Such an ion-exchange process occurring at an essentially fixed threshold voltage should, if allowed to persist, limit the advantages of the invention where large electrode spacings are required. It has been found however that the effect may be suppressed effectively by the introduction of any one of several different gases into the system at pressures from a few tenths of a micron to a few t of Hg. Air, H and A at similar pressures all produce essentially the same effect-the discharge disappears and the voltage applied to the system may be raised immediately and permanently to a higher value at which some factor other than gap discharge become a limitation.

At close electrode spacings, -i.e. one millimeter or less, the introduction of an inert gas at pressures up to several p. of Hg has no effect except to cause a substantial increase in the normal quiescent gap current. Essentially the same maximum field can be reached with or without gas pressure.

Thus if the apparatus of FIGURE 1 is to be operated at a voltage exceeding 300 kv., and with electrode spacings exceeding one millimeter, gas pressure within the vacuum enclosure 14 should be adjusted to be within the above specified range of values.

Referring now to FIGURE 4, a typical usage of the invention is shown, the apparatus being a parallel plate velocity spectrometer 21 of a type used in the study of nuclear phenomena. This apparatus is used to sort out nuclear particles of a selected velocity from a beam 20 of high energy particles having various differing velocities which beam is generally obtained from a particle accelerator 22 which might, for instance, be a synchrotron.

The spectrometer comprises a pair of spaced apart electrodes essentially similar to those hereinbefore described with reference to FIGURE 1 and thus includes a long rectangular negative glass electrode 12 having a conductive backing plate 13' disposed in a parallel spaced apart relationship with a fiat rectangular positive electrode 11', the electrodes having the specialized characteristics previously described. A pair of spaced parallel rectangular magnetic field coils 27 and 28 are disposed along the opposite sides of the electrodes 11' and 12 with the plane of the coils perpendicular to the electrodes and parallel to beam 26. A magnet power supply 29 is connected to the coils 27 and 28 so that current through both is in the same direction and a magnetic field is created between the two electrodes 11' and 12' at right angles to the electric field therebetween, and to beam 20, as shown by magnetic field lines 31. A high voltage power supply 32 has a negative voltage terminal connected to the backing plate 13' and has a positive voltage terminal connected to the electrode 11 establishing the electric field 33. A vacuum envelope 34 encloses the spectrometer and avacuum pump 36 is coupled thereto. To control the temperature of the glass cathode 12 for adjusting the resistivity thereof as hereinbefore described, an electrical radiant heating element 39 is disposed beneath the negative electrode and the back- 7 ing plate 13',- and is connected to a suitable source of current '41 through a variable resistor 42.

In operation, the beam 20 of accelerated charged particles is directed between the two electrodes 11 and 12- The electric field 33 tends to deflect the beam particles in one direction and the magnetic field 31 tends to deflect the beam particles in the opposite. For particles of a particular velocity the two deflection forces will equalize and such particles pass along the linear path 20 Without deflection while other particles are deflected and removed from the beam by striking the electrodes 11 or 12'.

Thus, from the input beam 20, there remains an output beam 20" comprised only of particles having the desired energy which energy is determined by the electrical and magnetic field intensities. The output beam 20" is directed toward a detector 38 which may be a bubble chamher for example. The intensities of the electric and magnetic fields are usually made adjustable for obtaining the particular magnetic and electric field strengths for separating any particles of a specific desired velocity.

The inclusion of the present invention in this type of apparatus allows a much more intense electric field to be obtained before sparking occurs. Consequently a substantial reduction in the physical size of the spectrometer may be obtained or the range of the spectrometer may be extended in comparison prior instruments of this class.

embodiment of the spectrometer having ten foot long electrodes, a gap spacing of five centimeters, a soft glass negative electrode operating at 450 kv. at a temperature of 100 C., and having an argon pressure of about one micron, has been operated in an 800 m.e.v./c. K beam generated by a proton synchrotron. Under normal conditions the spark rate varies between a few per day and a few per hour, much of which is attributable to sparks 'on the supporting insulators. After two months of operation, no damage to the electrodes had occurred. Operation of the spectrometer in a beamycomposed of particles of known charge mass and momentum provided an additional means for verifying that the expected electric field actually is present between the electrodes.

It will be apparent to those skilled in the art that nuinerous variations and modifications may be made within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In apparatus for providing an electrical field, the combination comprising a first and a second electrode disposed in spaced apart relationship, said first electrode being comprised of a material characterized by a high dielectric strength and by having a volume resistivity greater than one ohm-centimeter and a time constant of less than second, a conductor means contacting substantially the entire surface of said first electrode on the side thereof opposite from said second electrode, and means fior connecting said second electrode to a positive potential and said conductor means to a relatively negative potential. V

2. In apparatus for maintaining an intense electric field without excessive sparking, the combination comprising a first flat electrode fiormed of a material characterized by having a high dielectric strength and a volume resistivity greater than one ohm-centimeter and a time constant of less than l() second, a second electrode having a flat surface parallel to said first electrode and spaced therefrom to form a field gap, means providing a vacuum in said field gap, a highly conductive backing means contacting the surf-ace of said first electrode which is remote from said second electrode, and a high voltage power supply having a positive terminal connected to said second electrode and a negative terminal connected to said backing means.

3. In apparatus for producing an intense electric field, the combination comp-rising a first electrode and a second electrode spaced apart therefrom to forma field gap, said first electrode being formed of glass, a high voltage power supply having a negative terminal connected to said first electrode and having a positive terminal connected to said second electrode, and a vacuum enclosure surrounding said field gap.

4. In apparatus for producing an intense electric field substantially as described in claim 3, the further combination of means fior heating said first electrode to regulate the specific resistivity of said glass.

5. Apparatus for producing an intense electric field as described in claim 3 wherein said first electrode is comprised of soda-lime glass, and comprising the further combination of a heating element disposal proximal said first electrode for maintaining the specific resistivity thereof within the range 1 to 10 ohm-centimeters.

6. In a separator for selecting charged particles having a particular characteristic from a heterogeneous beam of particles, the combination comprising a pair of electrodes spaced apart to define a field gap along which said heterogeneous beam of particles may be directed, a first of said electrodes being comprised of glass having a specific resistivity exceeding one ohm-centimeter and having a bulk time constant less than 10* second, a high voltage power supply having a negative potential terminal connected to said first electrode and having a positive potential terrninal connected to the second of said electrodes whereby an electrical field is established between said electrodes, means providing a magnetic field between said electrodes which magnetic field is normal to said electric field and normal to said beam of particles, a magnet power supply connected to said magnetic field means, a vacuum tank surrounding said field gap, a vacuum pump connected with said tank, and means for heating said first electrode to bring the electrical characteristics thereofi within said specified ranges.

7. In apparatus for producing an intense electrical field, the combination comprising a first electrode formed of a material having a volume resistivity and time constant substantially equivalent to that of heated glass, 2. second electrode spaced apart from saidfinst electrode to form a field gap, a high voltage power supply having a negative terminal connected to said first electrode and a positive terminal connected to said second electrode, and a vacuum enclosure surroundingsaid electrodes and said field gap.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN APPARATUS FOR PROVIDING AN ELECTRICAL FIELD, THE COMBINATION COMPRISING A FIRST AND A SECOND ELECTRODE DISPOSED IN SPACED APART RELATIONSHIP, SAID FIRST ELECTRODE BEING COMPRISED OF A MATERIAL CHARACTERIZED BY A HIGH DIELECTRIC STRENGTH AND BY HAVING A VOLUME RESISTIVITY GREATER THAN ONE OHM-CENTIMETER AND A TIME CONSTANT OF LESS THAN 10-3 SECOND, A CONDUCTOR MEANS CONTACTING SUBSTANTIALLY THE ENTIRE SURFACE OF SAID FIRST ELECTRODE ON THE SIDE THEREOF OPPOSITE FROM SAID SECOND ELECTRODE, AND MEANS FOR CONNECTING SAID SECOND ELECTRODE TO A POSITIVE POTENTIAL AND SAID CONDUCTOR MEANS TO A RELATIVELY NEGATIVE POTENTIAL. 